In the simplest definition of the term, the diversity technique in the design of an antenna is a means of achieving reliable overall system performance through the use of an additional antenna. System performance may be characterized by signal strength, signal to noise ratio, data rate, error rate, or other factors. Of particular importance to wireless data systems is to achieve high data rates with low error rates.
In wireless or radio communications, particularly in mobile radio communications, antenna diversity is a well-known technique for mitigating fast fading caused by multipath signal propagation. In order to obtain diversity gain, at least two signals carrying the same information but with different fading characteristics (low signal correlation) are needed. Polarization diversity, which is one of the commonly used diversity techniques, utilizes the independent fading of vertically and horizontally polarized signal components. It is a potential technique for handsets, because large antenna separations are not required.
Thus, it is known that not only phase changes but polarization changes take place in a multipath signal environment, and therefore polarization diversity has been utilized to further improve the rate of data transfer in cluttered environments. Polarization diversity means the use of two antennas oriented such that in areas where antenna one has higher gain in one polarization, antenna two has higher gain in the orthogonal polarization. In general, polarization diversity is achieved by two antennas having orientation such that the polarization axes are approximately orthogonal. Furthermore, the main beams of the antennas can point to different directions and the antennas can be spaced some distance apart, in which case also angle and space diversities are utilized to decorrelate the signals. Again, by suitable signal processing, the average signal to noise ratio can be increased using polarization diversity compared to a single antenna system, as well as increased average signal to noise ratio compared to, for example, spatial diversity antenna systems.
One approach has used a planar inverted F antenna (PIFA) as an antenna applied in the polarization diversity technique. FIG. 1 depicts two square-shaped planar inverted-F antennas (100) (PIFAs or alternatively referred to as short-circuited patch antennas) positioned and fed as in FIG. 1 by a feed probe (104) and short circuit (105). The antennas (100) can be used to obtain good diversity gain (5–9 dB) in a personal mobile communication terminal (PMCT). This has been confirmed with radio channel sounder measurements. The size of a ground plane (103) is approximately 40 mm×100 mm (width×length). The size of antenna elements (101, 102) is approximately 16 mm×16 mm×7 mm (length×width×height).
However, the wireless devices tend to be designed for being, possibly, comfortably carried by the user. This trend has driven a need for designing relatively small, light and handy wireless devices for users.
This has for one's part created new challenges for antenna design and technique. For an example, compared to the typical internal antennas currently used in small PMCTs, such as mobile phones, the PIFAs of FIG. 1 are clearly larger, possibly, too large for the mobile phone application.
In view of various inherent limitations of antennas and wireless communications devices, it would be desirable to avoid or mitigate these and other problems associated with prior art. Thus, there is a need for a technique to provide more compact diversity antenna arrangement.